1. Field of the Invention
The invention relates to a forming method of a thermal insulation film which is formed on e.g. a wall surface of an internal combustion engine that is located in a combustion chamber.
2. Description of Related Art
An internal combustion engine, such as gasoline engine, diesel engine, and the like, is mainly composed of a cylinder block, a cylinder head, and a piston, and its combustion chamber is delimited by a surface of a bore of the cylinder block, a top surface of the piston inserted in the bore, a bottom surface of the cylinder head, and top surfaces of an intake valve and an exhaust valve provided in the cylinder head. As high output from the internal combustion engine is increasingly demanded recently, it becomes important to reduce its cooling loss. As one of the solutions to reduce the cooling loss, a method may be proposed in which a thermal insulation film formed by ceramics is formed on an inner wall of the combustion chamber.
However, the above ceramics generally has low thermal conductivity and high thermal capacity, thus steady rise of the surface temperature may incur reduction of intake efficiency and knocking (abnormal combustion caused by heat accumulated in the combustion chamber), and therefore the material as the thermal insulation film for the inner wall of the combustion chamber has not been widely used.
Therefore, it is desirable that the thermal insulation film formed on the wall surface of the combustion chamber is formed by material which not only is heat resistant and heat insulative, but also has low thermal conductivity and low thermal capacity. That is, in order to lower the temperature of the wall surface by following temperature of fresh gas during the intake stroke, it is preferable to have low thermal capacity, so that the temperature of the wall surface would not be steadily raised. Moreover, in addition to the low thermal conductivity and low thermal capacity, it is also desirable that the thermal insulation film is formed by material which can withstand explosion pressure in the combustion chamber upon combustion, injection pressure, and repeated stresses caused by thermal expansion and thermal contraction, and has high adherence with base material of the cylinder block and the like.
Here, attention is directed to published prior art. Japanese Patent Application Publication No. 58-192949 (JP 58-192949 A) discloses a piston and a manufacturing method therefor, wherein an alumite layer is formed on a top surface of the piston, and a ceramic layer is formed on a surface of the alumite layer. With this piston, its heat resistance and thermal insulation are made excellent by forming the alumite layer on the top surface.
As such, by forming the alumite layer (anode oxidation coating film) on a wall surface of an internal combustion engine that is located in the combustion chamber, it is possible to form an internal combustion engine having excellent thermal insulation, low thermal conductivity, and low thermal capacity. Also, in addition to these properties, it has excellent swing property which is also important performance required by an anode oxidation coating film. Here, “swing property” means that the anode oxidation coating film has thermal insulation performance and its temperature follows temperature of the gas in the combustion chamber.
When being observed microscopically, the above anode oxidation coating film takes a structure having a plurality of adjacent cells, and has a lot of cracks on its surface, wherein a portion of the cracks extend inwardly (that is, extend in thickness direction or approximately in thickness direction of the anode oxidation coating film). There are also lots of internal defects within the film that extend in directions other than the thickness direction (horizontal direction orthogonal to the thickness direction or approximately the horizontal direction). Moreover, it is known that these cracks and internal defects are micro-pores each having a diameter (or maximum diameter in cross sectional dimensions) of micrometer-scale approximately ranging from 1 μm to several tens of μm. Furthermore, the “cracks” stem from crystalline matter of aluminum alloy for casting.
Moreover, inside the anode oxidation coating film, in addition to the above cracks and internal defects of micrometer-scale, there are also many small pores each having a diameter of nanometer-scale (nano-pores), and generally, the nano-pores are also present in a state where they extend from the surface of the anode oxidation coating film in its thickness direction or approximately in the thickness direction. Furthermore, the “nano-pores” stem from anode oxidation treatment and are arranged regularly.
As such, the formed anode oxidation coating film generally includes therein micro-pores such as surface cracks, internal defects or the like having a diameter or maximum dimension in cross-section of micrometer-scale, and a plurality of nano-pores of nanometer-scale.
However, if the surface roughness of the thermal insulation film constructed by the above anode oxidation coating film is large, abnormal combustion may be easily incurred, resulting in degradation in fuel efficiency. Therefore, in order to lower the surface roughness of the thermal insulation film constructed by the anode oxidation coating film, generally the surface is abraded. At this time, since the anode oxidation coating film has a plurality of micro-pores therein as described above, there is the issue that the smoothness of the surface of the thermal insulation film cannot be improved due to appearance of internal micro-pores on the surface even after repeated abrasion. This will be described with reference to FIG. 10 and FIG. 11.
As shown in FIG. 10, a thermal insulation film M constructed by an anode oxidation coating film is formed on a wall surface W of a cylinder block, etc. constituting an internal combustion engine. The thermal insulation film M has a plurality of micro-pores Pm each having a diameter dm of micrometer-scale and a plurality of nano-pores Pn each having a diameter do of nanometer-scale. Although the micro-pores and the nano-pores are exposed at the surface of the thermal insulation film, since in particular the micro-pores Pm having a larger diameter dm are exposed, the surface roughness becomes large. Therefore, even if the surface is abraded in order to improve its smoothness, as shown in FIG. 11, the smoothness of the surface cannot be improved so long as the micro-pores Pm inside the thermal insulation film M are exposed.
Here, in Japanese Patent Application Publication No. 2012-72745 (JP 2012-72745 A), a thermal insulation structure is disclosed, wherein a porous layer is formed on a surface of a base material made from aluminum alloy by anode oxidation treatment, and a covering layer having lower thermal conductivity than the base material is provided on the porous layer. By way of the anchor effect brought by surface unevenness of the porous layer, adherence of the porous layer with the covering layer is improved. However, since the surface of the porous layer (anode oxidation coating film) has unevenness, despite the covering layer provided on the porous layer, the surface unevenness may be largely reflected at the surface of the covering layer, so surface roughness of the thermal insulation film constructed by the porous layer and the covering layer cannot be improved.